In recent years, for glass products used in the field of optical communication or in the semiconductor industry or the like, a very strict control is carried out with respect to their minor impurities and fine bubbles in the products. For example, in a case where the glass product is a crucible for pulling up a silicon single crystal, if it contains minor impurities, the minor impurities eluted from the crucible will be entrapped in the silicon single crystal and deteriorate the performance of the semiconductor. Especially, Group III-V elements such as phosphorus and boron substantially influence the semiconductor properties, and a special attention is paid to them. Further, in the case of a quartz jig to be used at a high temperature, Group III-V elements contained in the shaped product of quartz will diffuse and contaminate a silicon wafer, whereby the desired semiconductor properties can hardly be obtained. Further, if the crucible contains fine bubbles, there will be a problem such that they not only cause disturbance of the liquid surface during pulling up of a silicon single crystal and formation of crystal defects, but also deteriorate the durability of the crucible.
Such high quality glass is produced mainly by e.g. (1) a method of purifying natural quartz, (2) a method in which a fume generated by decomposition of silicon tetrachloride in an oxyhydrogen flame is deposited and grown on a substrate, or (3) a method in which a silica gel obtained by e.g. hydrolysis and gelation of e.g. a silicon alkoxide, is baked and a synthetic quartz powder thereby obtained is employed.
However, the method (1) has a problem that there is a limit in reducing the content of minor impurities, and the method (2) has a problem that the production cost is extremely high. On the other hand, in the method (3) wherein a silica gel, particularly a silica gel derived from a silicon alkoxide, is employed, it is possible to obtain a synthetic quartz powder having relatively low content of minor impurities, but the desired level is not necessarily satisfied. Further, by this method (3), fine bubbles are likely to form in the shaped product as the final product, thus leading to the above-mentioned problem caused by fine bubbles.